The present invention concerns a milling machine for emulsifying immiscible liquids and, more particularly, a suitable mill for the continuous production of high phase ratio emulsions of a hydrophobic fluid within a hydrophilic fluid, having a uniform particle size.
The most immediate and specific application of this invention is in making emulsions for the preparation of microcapsules of a more uniform size for use in the manufacture of carbonless copy papers. The typical milling machines for emulsifying oil-in-water for use in manufacturing microcapsules, involve multiple recirculatory milling procedures. Such milling machines offer a variety of fluid pathways involving greatly varying shear fields, excessive hold up volume, and excessive dead space. Furthermore, the oil is introduced progressively, over a very wide range of phase ratio conditions during recirculation of the emulsion. Such multiple pass milling procedures invariably produce microcapsules having a very broad size distribution wherein many capsules are larger than or smaller than an optimal size.
In general, regardless of the encapsulation chemistry, overly small capsules are inefficient. They tend not to be broken by the application of imaging pressure. When they do rupture there is little sheet-to-sheet transfer of core oil. Overly large capsules, on the other hand, cannot be adequately protected from incidental rupture. They either break spontaneously when the paper is wound or they subsequently diminish the resistance of the paper to scuffing when handled. Thus the undersized and oversized microcapsules both degrade the performance and increase the cost of the product.
When using capsule wall formation based upon the interfacial polymerization of certain isocyanate compounds which have been pre-dissolved in the core oil, the wall structure begins to form substantially immediately upon exposure of the oil droplet to the external phase. After initial droplet formation, multiple pass milling may disrupt the fragile pre-wall. This could result in a broad size distribution which is undesirable. Also, the wall thickness of these microcapsules is directly proportional to the size of the core oil droplet. Overly small capsules are very weak, having a tendency to be leaky (i.e., permeable) and subject to incidental damage. Overly large capsules, though stronger, cannot be protected by the stilts in the coating matrix. For any given product, then, there is a preferred capsule size which represents the optimum balance between overall performance and cost.
Accordingly, a need exists for an improved milling machine that significantly eliminates the size distribution problems associated with the previous emulsifiers and milling machines.